I toured until last year on an older bike, 1973, a racing bike modified for touring. If the Miyata is in good condition, it is certainly good enough for long distance touring.
In spite of saying that, if you have to ask, the bike may not be right for you. You should be well versed with basic maintenance to have completely overhauled the bike before you start on a long distance tour, be able to able to do basic repairs on road, have a least a minimum set of tools with you, and know where to get the no longer manufactured parts you may need. Not that a bike that old in good condition to start with, completely overhauled before commencing trip, will give you appreciably more trouble than a newer bike, but you never know.